The objective of the proposed research is to increase our understanding of a rarely reported disorder characterized by decreased leukocyte myeloperoxidase activity. This should provide increased knowledge of normal and abnormal leukocyte physiology. Despite the fact that myeloperoxidase accounts for 1-5% of the dry weight of normal polymorphonuclear leukocytes, its importance is unknown. This is evidenced by the fact that deficient subjects have been reported who have no apparent increase in clinical disease. However, there have been very few cases reported and only one small family with familial myeloperoxidase deficiency has been studied. Thus many questions remain unanswered such as: What is the mode of inheritance? Is the familial myeloperoxidase deficiency associated with an abnormal enzyme or a decreased quantity of the normal enzyme? What is the importance of the enzyme deficiency? Are there any other associated genetic traits suggesting linkage? In the course of routine hematology work at the University Medical Center using an automated leukocyte differential counter which incorporates myeloperoxidase strains, we have detected six probands with MPO deficiency in the past year. Three of these probands are members of large Mormon families providing the opportunity to study large numbers of myeloperoxidase deficient subjects. Preliminary studies have shown several myeloperoxidase deficient members in two of the families giving us the potential for more extensive investigation of the above questions.